Bacillus thuringiensis, Bt, is a Gram-positive rod-shaped bacterium widely existing in various ecological environments. The most notable characteristic of the species is to form hypopus of endogenous spore during the growth cycle thereof whilst generating parasporal crystal consisting of insecticidal crystal proteins (ICPs) having specific toxicity towards a wide variety of insects. These insecticidal crystal proteins have specific bioactivity on over 500 types of insects of 10 orders of insecta such as Lepidoptera, Diptera, Coleoptera, Hymenoptera, Homoptera, etc. and some pests in Protozoa, Nemathelminthes, Platyhelminthes and so on (Schnepf H E, Crickmore N, Rie J V, Lereclus D, Baum J, Feitelson J, Zeigler D R, Dean D H. 1998. Bacillus thuringiensis and its pesticidal crystal proteins. Microbiol Mol Biol Rev, 62: 775-806). Bacillus thuringiensis is widely applied to the development of biological insecticides as it has high insecticidal activity on various types of insects and the advantages of no harm to environment and human and livestock and so on. The biological insecticides developed on the basis of Bacillus thuringiensis has been the most successful biological agricultural chemical up to now, occupying more than 90% market of the whole industry of biological agricultural chemical and widely used for the control of pests in agriculture, forestry and storage industry. At the same time, people continue transferring genes of insecticidal crystal proteins of Bacillus thuringiensis into plants so as to construct transgenic plants for the control of agricultural pests.
Bacillus thuringiensis insecticides initially were produced by using the screened wide-type strains. Despite many advantages of Bacillus thuringiensis, the field applications thereof still lead to the problems of unstable control effects, short residual period, slow rate of killing pests and insensitivity to some target pests and so on. As molecular biology is flourishing, people start to genetically modify wide-type strains by means of genetic engineering to improve the insecticidal activities of the initial strains and prolong the residual periods thereof. However, while the applications of Bacillus thuringiensis insecticides are continuously spread and the application intensities are increasing, scientists continuously find the phenomena of the chemical resistance of target pests. These factors seriously limit the applications and popularization of insecticide products of Bacillus thuringiensis and are a menace to the rapid development of the whole industry. One effective solution to the problem is to find new insecticidal effect-enhancing factors for remarkably improving the insecticidal activities of the initial strains and postponing or overcoming the resistance of insects thereon.
Recently, searches for new insecticidal effect-enhancing factors have been the most prosperous part in the field of the applications and researches of Bacillus thuringiensis. Hilder et al. increased the insecticidal activities of the insecticidal crystal proteins generated by Bacillus thuringiensis by over 20 times by using serine protease inhibitors (Hilder V A, Gatehouse A M R, Sheerman S E, Barrer R F, Boulter D. 1987. A novel 4 mechanism of insect resistance engineered into tobacco. Nature 330:160-163). Regev et al. found that chitinase could dramatically increase the insecticidal activities of the insecticidal crystal proteins of Bt by degrading chitin layers of peritrophic membranes of insects (Regev A, Keller M, Strizhov N. 1996. Synergistic activity of a Bacillus thuringiensis delta-endotoxin and a bacterial endochitinase against Spodoptera littoralis larvae. Appl Environ Microbiol 62:3581-3586). After that, more and more synergistic effect-enhancing factors were discovered, such as zwiitermicin A, chaperone P20, bacteriolysin Cyt, cadherin fragment CR12—MPED etc. (Pardo-López L, Mũnoz-Garay C, Porta H, Rodrírguez-Almazán C, Soberón M, Bravo A. 2008. Strategies to improve the insecticidal activity of Cry toxins from Bacillus thuringiensis. Peptides. In press). These research results provide many very effective strategies to improve the insecticidal activities of Bacillus thuringiensis strains, or excellently postpone or overcome the resistance of insects thereon, thereby enormously promote the development of the industry of Bacillus thuringiensis insecticides and of the field of transgenic plants based on Bt.